The present invention relates to large rotating electrical machines equipped with salient pole rotors such as synchronous motors used in industry and synchronous generators used in hydro power plants. The invention relates particularly to a design for controlling the cooling air flow in the rotor by choking the air flow in the air gap and sealing the gaps between adjacent poles.
Synchronous machines with salient pole rotors are normally cooled with air. Large such machines have radial cooling ducts in the stator. The air is delivered to the stator ducts by first passing axially through the pole gaps in the rotor. In the pole gaps, the air passes axially along the field windings, thereby cooling them. The relatively high peripheral speed of the rotor causes ventilation losses which consist partly of air gap friction losses and partly of a component which is proportional to the mass flow of air.
In order to minimize the ventilation losses it has been suggested in U.S. Pat. No. 3,157,806 that the field winding be water cooled and the pole gaps completely filled with non-magnetic filling compound or enclosed by sealing members. This has the advantage of making the rotor surface smooth and cylindrical which reduces air gap friction. Several hydro generators have been built according to this concept and are described in xe2x80x9cFully water-cooled 190 MVA generators in the Tonstad hydroelectric power stationxe2x80x9d, Brown Boveri Review, (1969) 8, p 380-385. In this concept both the stator and rotor are water-cooled which means that the mass flow of air through the air gap can be minimal, thus reducing ventilation losses even more. However, water cooling is expensive and technically complex, especially in the rotor. For this reason it is advantageous if the rotor is air-cooled while maintaining a cylindrical rotor surface. In U.S. Pat. No. 4,274,027 it is suggested to seal the gaps between adjacent poles but to leave openings at opposite ends of the pole gaps, obviously for axial air cooling of the field windings. However, it is not further described how the machine is cooled. Assuming that the stator is water-cooled, a drawback with this construction is that in the air gap between the stator and the rotor, an air flow will pass that serves no purpose, it only contributes to the ventilation losses.
The object of the invention is to provide air flow control by an air seal placed in the air gap between stator and rotor. The device reduces the mass flow of air that enters axially into the air gap. Used in conjunction with pole gap seals this will reduce the ventilation losses. Advantageous further developments of the invention are indicated in the following description.
The present invention applies to a machine with a stator without radial ventilation ducts, e.g. water-cooled, and air-cooled rotor in which the pole gaps of the rotor are covered, thereby also blocking the entrance to the air gap on the fan side so that a larger amount of air from a rotor fan supplying the air gap of the rotor, is blown into the pole gaps. This means that a lower flow volume is required than if the air is divided into one air flow through the air gap and one air flow through the pole gaps.